Covered stents for implantation into a body vessel, duct or lumen generally include a stent and a cover attached to the stent. A porous structure of the cover, depending on the porosity, may enhance tissue ingrowth after the covered stent has been implanted. A porous structure affixed to an implantable device also may serve as a reservoir for bioactive components and/or reduce embolization by trapping thrombus against a vessel wall.
Porous membranes for use in medical devices are known in the art. For example, U.S. Pat. No. 4,759,757 to Pinchuk describes the formation of a porous membrane by leaching water soluble inorganic salts incorporated into the membrane to create pores where the salt crystals were initially located. U.S. Pat. No. 6,540,776 to Sanders Millare et al. describes a perforated membrane in which a pattern of interstices is created by removing material, for example, by laser cutting. The foregoing manufacturing methods require at least two process steps to form a porous membrane.
One step processes for forming porous membranes also are known in the art, for example, using spinning techniques. U.S. Patent Application Publication No. 20040051201 to Greenhalgh et al. describes an electrospinning process in which a membrane is formed from a plurality of randomly-oriented, intertangled, non-woven fibrils.
Spinning techniques that produce less random, but non-uniform membranes, also are known. For example, U.S. Pat. No. 4,475,972 to Wong describes a porous polymeric material made by a process in which polymeric fibers are wound on a mandrel in multiple overlying layers. The fibers contain unevaporated solvent when deposited in contact with one another, so that upon evaporation of the solvent the fibers bond together. The fibers laid in one traverse are wound on the mandrel parallel to each other and at an angle with respect to the axis of the mandrel. In the next traverse, the angle of winding is reverse to the previous angle, so that the fibers crisscross each other in multiple layers to form the porous structure.
U.S. Pat. No. 4,738,740 to Pinchuk et al. describes a spinning method similar to that of Wong and further comprising intermittently applying a electrostatic charge to ensure reattachment of broken fibers to the mandrel. U.S. Pat. No. 5,653,747 to Dereume describes a vascular graft with an expandable coating produced by the spinning technique of Wong and having pores that open when the tubular support member expands.
All of the foregoing spinning processes suffer from an inability to tightly control the pore size and pore pattern of the resulting membranes. More specifically, lateral deviation of the fibers using previously known spinning techniques has resulted in unsteady collocation of the fibers and the need to deposit multiple layers to ensure adequate coverage. Consequently, previously-known techniques produce either stiff membranes formed of multiple layers and unsatisfactory porosity, or porous, elastic membranes with insufficient strength.
In view of the foregoing, it would be desirable to provide membranes having controlled porosity, pore pattern and pore distribution.
It further would be desirable to provide a one step manufacturing process to produce membranes having controlled porosity, pore pattern and pore distribution.
It still further would be desirable to provide a one step manufacturing process to produce membranes having controlled porosity and/or pore pattern wherein the membrane includes a bioactive substance that may be eluted from the membrane after implantation.
It also would be desirable to provide manufacturing processes to produce membranes having the desired porosity, pattern and distribution characteristics for use in medical implants.